MIT photographer tells a comet's tale

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Viewed from the city with the naked eye, comet Hale-Bopp appears as little more than a smudge in the northwest sky. But with the right equipment and know-how, the comet is a truly spectacular sight. Steve Slivan, a postdoctoral associate in earth, atmospheric and planetary sciences, has both. He provided MIT Tech Talk with this explanation of how he made two of his photos (see page 8 for the other) and what they show.

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Comets are scientifically interesting because they're thought to be the surviving ancient remnants of the processes that ultimately led to the formation of our solar system. It has also been hypothesized that comets may have been an important source of material for Earth's atmosphere, and perhaps also were responsible for bringing to early Earth the organic molecules needed for the development of life.

These images of comet Hale-Bopp were recorded on April 9 at the Wallace Astrophysical Observatory, MIT's teaching observatory in Westford, MA. That evening was exceptionally clear and cold (19' F) with little wind, providing ideal conditions for astrophotography.

Both exposures were made on Kodak Technical Pan 2415 film, which produces extremely high-resolution black-and-white negatives. To increase this film's sensitivity to the low light levels of the comet's fainter features, the film was gas-hypersensitized by being baked in a moisture-free environment for two hours before being loaded into the camera.

To capture the comet against the backdrop of the Milky Way in the constellation Perseus, the camera rode "piggyback" on a telescope tracking with Earth's rotation during the exposures. Since Hale-Bopp also moves slowly with respect to the background stars, small guiding corrections needed to keep the camera pointed precisely at the comet were manually made continuously during each exposure.

The first photo has a field of view about 25 by 30 degrees, and was recorded using a regular 50mm lens at f/2 for an eight-minute exposure. North is toward the upper-right. Both the bright fan-shaped dust tail and the fainter narrow ion tail of the comet appear in the photograph. The dust tail is illuminated only by reflected sunlight, while the ion tail glows with the emissions of carbon monoxide ions.

The coma at the head of Hale-Bopp, perhaps several hundred thousand kilometers across, is overexposed in the photograph and hides the nucleus of the comet buried at its center. This nucleus is about 30 kilometers in diameter and is the source of the dust grains and ions making up the tail. Comet nuclei appear to made up of mostly water ice and dust, and for this reason are often described as "dirty snowballs."

The contrast of the ion tail in the photograph was improved by using a broadband light-pollution filter over the lens. This special type of interference filter reduces unwanted background sky brightness caused by poorly shielded mercury and sodium streetlamps. It is this light pollution that unfortunately renders the outer coma and ion tail invisible when viewing the comet from any urban area such as the MIT campus.

Along with the comet, this photo also reveals silhouettes of the trees surrounding Wallace (along the bottom of the picture), a section of the Milky Way (which runs across the field from the upper-left to the lower-right corners), the close pair of bright open star clusters known as the "Double Cluster" (near center right), and the dim "California Nebula," an emission nebula of glowing hydrogen (just above the tip of the tall tree along the left edge).

CLOSER VIEW

The second photo shows a field of view about 3 by 5 degrees, and was recorded using a 400mm lens at f/5 for a 15-minute exposure. North is toward the top. The magnified image scale shows more detail in the structure of the ion tail near the head. The bright star to the left of Hale-Bopp is the variable star Algol, an eclipsing binary system.

Careful examination of this photo reveals that the background star images are actually short streaks, which result from the manual guiding and indicate the direction of the comet's motion.

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Dr. Slivan teaches astrophotography as part of the fall-term subject 12s23/12.409 (Hands-on Astronomy: Observing Stars and Planets), which also covers visual observing and electronic imaging. For more information and sample images, see the Web pages at .

A version of this article appeared in MIT Tech Talk on April 16, 1997.